Recent microprocessor designs use a flip-chip assembly to improve power distribution and achieve higher operating frequencies. Debug probing of such devices relies on what is known as Laser Voltage Probing (LVP). However, LVP technology cannot accurately measure edge delays for multi-GHz frequencies and the laser invasiveness is increasing with smaller transistor geometries.
To overcome these problems, methods to translate signal edge timing information into light emission that can be accurately measured using a Time-Resolved Emission (TRE) or InfraRed-Emission Microscope (IREM) have been proposed. These methods are based on the phenomenon that hot electrons in a saturated NMOS transistor (or beacon device) emit infrared radiation both under static bias and switching condition. See T. Eiles, et. al., “Optical Probing of Flip-Chip Packaged Microprocessors”, ISSCC Digest of Technical Papers, pp. 220-221, February 2000, and L. T. Hoe, et. al., “Characterization and Application of Highly Sensitive Infra-Red Emission Microscopy for Microprocessor Backside Failure Analysis”, Proceedings of the 7th IPFA, pp. 108-112, 1999. Thus, as indicated in FIG. 1, infra-red light is emitted from an NMOS transistor 10 when in saturation, i.e., Vds>Vgs−Vt.
J. C. Tsang et al., in “Picosecond hot electron emission from submicron complementary metal oxide semiconductor circuits,” Appl. Phys. Lett., p.889-891, February 1997 describes using a commonly available, very low noise optical detector such as mercury cadmium telluride detector array, which has good sensitivity in the range of 0.9-1.45 μm, one can measure the emission intensity (Iemission) accurately. The use of light emission for time-dependent analysis is described by Dan Knebel et al. in “Diagnosis and Characterization of Timing-related Defects by Time-dependent Light Emission”, International Test Conference, p. 733-739, August 1998. This paper describes clock skew analysis as one of many potential applications. In addition, it suggests the use of a phasedetector circuit (PFC) to modulate the duration of light pulse as a function of skew.
Thus, as shown in FIG. 2, in the prior art, a Phase-Frequency Comparator (PFC) 11 is used to focus the mode of operation on one particular edge (i.e. rising edge) for which a timing delay Δt is to be measured. The PFC is coupled to a saturated NMOS transistor (or beacon device)13 which emits infrared radiation. The radiation is then detected by a photon detector 15, which may be a TRE or IREM as noted above. The resulting measured pulse, has a width representing Δt.
However, we have found that this method is limited by a ‘deadband region’ where, if the skew is less than the rise/fall time of the clock under test, it will go undetected. A need, therefore, exists for a method and apparatus which overcomes this limitation.